1. Technical Field
The present invention relates to sheet inspection systems of the type used in measuring quality attributes of a moving sheet of material during its continuous manufacture. More particularly, the invention pertains to those systems which derive such measurements from prior measurements of physical properties of the sheet, which prior measurements are based on the interaction of one or more selected wavelength bands of radiant energy with one or more constituents of the sheet. Yet more particularly, the invention concerns those systems which must repeatedly produce such measurements for each of a plurality of measurement zones (also known as "data boxes").
2. Description of the Background Art
Systems of the above description have been in use for many years. In a typical arrangement, one or more sensors mounted on a conventional sheet-traversing structure move back and forth across the entire cross-machine width of the sheet in order to provide for measurement in each measurement zone. This reciprocating motion is commonly referred to as "scanning",
The sensors typically employ a source of radiant energy. Radiation directed from the source and interacting with the sheet is detected by a detection system which typically produces analog signals indicative of one or more physical properties of the sheet. Examples of such physical properties are transmittance, reflectance, and flourescence. The physical property is in turn indicative of one or more quality attributes. Examples of such quality attributes are basis weight, moisture content, color, degree of cure, thickness, gloss, and ash content.
For some quality attributes, it is unnecessary to design the sensor to detect radiation at two or more selected ranges of wavelength (hereinafter "wavelength bands"). In measuring gloss, for example, a source of visible light is used with a suitable source/detector geometry, and a measurement of gloss is produced from the reflectance of a relatively broad range of visible light wavelengths without regard to the reflectance of narrower bands within the visible range. However, the ability to measure other quality attributes such as color, moisture content, polymer content and weight per unit area may be based upon independent detection of a plurality of selected wavelength bands with one or more constituents of the sheet. In measuring color, for example, it is necessary to resolve light which has interacted with the sheet into a plurality (typically, at least sixteen) of wavelength bands and to measure the physical property for each of the bands before the physical property measurements are combined to produce a single measurement of color.
As mentioned above, the sensor typically moves in reciprocating fashion across the entire cross-machine width of the sheet. The time required for the sensor to traverse the entire width may be on the order of fifteen to sixty seconds. In comparison, the sheet may be moving in the machine direction at speeds in excess of 3,000 feet per minute. Consequently, between successive appearances of a sensor in a particular measurement zone, several hundreds of feet of production may have passed the sensor without measurement in that zone. Moreover, in the time it takes the sensor to travel from one zone to the next, over 50 feet of production may have passed given the above speeds. Thus, the line of measurement for a particular scan may be viewed as a very long diagonal extending from one edge of the sheet to the other.
One of the technical problems thus presented is that when changes in a quality attributes are observed, it is difficult to know the extent to which the changes result from MD (machine direction) variation of the attribute as opposed to CD (cross-machine direction) variation. This poses a problem in controlling the quality attribute in that the steps taken to correct for MD variation differ markedly from those taken to correct for CD variation. Thus, in order to immediately implement the right combination of control steps, the precise nature of the variation must be known. Since the motion of the sensors precludes knowledge of the nature of the variation at any particular time, current control schemes employ assumptions respecting the directional nature of the variation in order to calculate optimal combinations of control steps. These calculations are repeatedly updated to reflect the most recent measurement data for the quality attribute of interest.
In addition to the foregoing problem, there are various maintenance and precision problems associated with the mechanical motion necessitated by conventional scanning sensors.
Accordingly, in the field of measurement and control of continuously-produced sheet materials wherein measurement is based on the interaction of one or more selected radiation wavelength bands with the sheet, and particularly in the area of papermaking, there has been a continuing need for sensors which can provide more rapid measurement of quality attributes in each measurement zone of the sheet, and for sensors which can do so without resort to the conventional scanning motion.
It is an object of this invention to provide a non-scanning sensor for use in measuring, in each measurement zone of a moving sheet of material, a selected quality attribute of the sheet.
It is another object of this invention to provide a nonscanning sensor that is suitable for applications wherein measurement is based on the interaction of one or more radiation wavelength bands with one or more constituents of the sheet. It is a further object of this invention to provide such a sensor that is operable with a single detector unit so that conventional approaches employing plural detectors or time-multiplexing of radiation transmissions are unnecessary.
Yet another object of this invention is to provide, in systems which use scanning sensors, significant improvement in the ability to separate observed variations in a quality attribute into MD and CD components.